If you want that all your messages are shown horizontally centered, just activate the centered alignment in the "text" field inside the message window (See Creating a Message Replacement).
If you want a particular message to be centered, then you specify the global variable NMGLO:Align as 'C' just before calling the Message() function. For example, like this:
NMGLO:Align = 'C' !Centered
Message('What are you waiting for?|Buy me now!||(This message is auto-centered)','Act now')
Will show as:
Note: This feature will not work in Clarion 6.2 9047. There is a bug in Clarion 6.2 (up to 9047) that prevents it to work at all. It was fixed in Clarion 6.2 9048.
Old (version 1.x) method:
If you want a particular message to be centered, then you can add the keyword @c@ at the beginning of your message. For example, the message
Message('@c@What are you waiting for?|Buy me now!||(This message is auto-centered)','Act now')
Will show the same as above.